Problem reading TTL serial data from weather station

Projects Networking, Protocols, and Devices
1

This is supposed to be easy, but I've been banging my head on it. Please help me figure out what I'm doing wrong!
I've got a weather station that sends out data at 2400 and I'm using an Arduino Mega 1280 board. When the weather station is hooked up to an old usb/serial converter I've got then I can capture the correct data on a PC, but when I connect it directly to the Mega's RX2 (in this case) I can't make sense of the data.
I've got the Mega programmed with a simple pass-thru serial program that reads from Serial2 and writes to Serial to try and see what's going on.
Trial and error along with a DSO showed that a "signal indicator" LED w/1K resistor on the line is needed to get a good square wave. Without that little circuit, the Mega just sees junk, with that circuit it sees repeating junk, which is only arguably better!

Here's what I should see (and do see on the PC):

!!00000080----0000----------------0000005A00000000

Here's what I see on the Mega instead (byte values, comma delimited):

111,111,246,246,246,246,246,246,159,165,165,165,165,159,159,159,159,165,165,165,165,165,165,165,165,165,165,165,165,165,165,165,165,159,159,159,159,159,159,159,159,159,159,159,159,159,159,159,159,229,235,0

There's no obvious logic that could change the 111 into 33 (ascii '!') and also change the other digits into 48 (ascii '0'), and I really don't know what else to try.

Any help is appreciated.

2

Well I've managed to figure out that the signal has to be inverted (couldn't find anywhere that just came out and said that), by rigging a very simple NPN transistor circuit with a pull up resistor on the collector output. This lets the Arduino see the right values, but it doesn't have the best wave form on a DSO. I tried a cmos CD4049 hex inverter and a SN7404 TTL hex inverter, but couldn't make either of those work.
I've tried a couple MAX3232 converters but the 5V TTL level semi-RS232 input from the weather station doesn't seem to be strong enough to get a useful output.
Any suggestions?

3

My suggestion is to use the transistor.
With a resistor of 10k to the base and 4k7 or 10k at the collector.

If the signal from the weather station is very dirty, you could add a diode to the base resistor (if the signal is noisy when low) or change the resistor in the base or a filter with a capacitor.
Are the grounds connected properly ?

2400 baud is slow, that should not be a problem.
Can you measure the baudrate ? Perhaps the weather station is not precise 2400. If so, you could adapt the Arduino Mega's baud rate.